The invention described herein is generally related to hard metal compositions, also known in the trade as hardmetals, such as are used in machine tools, rock drilling bits and in other applications requiring metal components having high hardness. More particularly, this invention is related to the tungsten and molybdenum boride-carbide family of hardmetals.
Tungsten carbide is a well known hard material, having a hardness value of 92 to 96 as measured by the Rockwell A hardness test. However, pure tungsten carbide is too brittle for use in most applications. Accordingly, it is well known to combine tungsten carbide with a relatively soft binder metal, such as cobalt, nickel, iron or a mixture thereof, to make a hard composition having both high hardness and high fracture toughness. The compositions utilizing cobalt as the binder are generally recognized as being superior in the characteristics which are important in practical applications, primarily hardness and toughness, and for this reason the cobalt-tungsten carbides are widely used in rock drilling bits, tool bits, tire studs and similar applications. Cobalt, however, is a metal which is almost exclusively imported into the United States, with the result that the availability and price of cobalt are unstable and the price is subject to wide and unpredictable fluctuations.
Boron carbide is second only to diamond and cubic boron nitride in hardness. However, its practical utility is limited by its high brittleness and virtual lack of elongation. Some efforts have been made to react boron carbide with various metals to form metal carbide/metal boride compositions which are both hard and resistant to fracture, so as to render them desirable as machine tool bits and the like. For example, U.S. Pat. No. 3,386,812 discloses a nickel-boron carbide composition. More important in this regard, however, is the applicant's own previous work, which is disclosed and claimed in the U.S. Pat. No. 4,400,213. In that application there is disclosed the applicant's development of a hard composition which consists generally of the hot-pressed reaction product of either tungsten or molybdenum mixed with boron carbide and a binder of nickel and/or iron. The metallic component of that composition (i.e., the non-boron carbide component), as disclosed and claimed in the referenced application, contains tungsten or molybdenum in a concentration of at least 90 percent by weight. Consequently, the hard composition has a relatively high density, on the order of 15 to 16 g/cc. Although such a high density is of no consequence in many applications, and in fact constitutes an advantage in some applications, there are other applications in which it would be desirable to employ similar hard metal compositions having a relatively lower density. Such applications include, for example, machine tools having high speed moving tool bits, hard metal components used in aircraft, hard metal components used in munitions projectiles, and tire studs.